Transvaginal ultrasonography for all placentas that appear to be low-lying or over the internal cervical os.

The purpose of this study was to determine in what percentage of cases the assessment of placental localization using transabdominal sonography (TAS) was changed after transvaginal sonography (TVS) was applied. TVS was prospectively performed on all pregnant women of at least 15 weeks' gestation, when the placental edge using TAS appeared to be over or within 2 cm (low-lying) of the internal cervical os. The time required for the TVS scan and the distance of the placental edge from the internal cervical os were recorded. Of the 168 patients entered into the study, 131 were analyzed. Landmarks were poorly seen in 50% of the cases when using TAS. In 66 cases, the placenta appeared low or possibly over the internal cervical os using TAS, but a definitive diagnosis could not be made due to suboptimal visualization. In the remaining 65 cases, visualization of the internal os and placental edge was possible using both TAS and TVS. In this group, there was a change in the diagnosis in 26% of the cases after TVS was performed. Our results suggest that optimal visualization of the placental edge and internal cervical os is usually difficult with TAS when the placenta appears low-lying or over the internal cervical os. The assessment of placental localization was changed in over one-quarter of cases (26%) after transvaginal sonography was performed. The use of transvaginal ultrasound should be seriously considered when the placenta appears to be low or over the internal cervical os by transabdominal ultrasound.